This invention relates to a method of bonding an inner lead of a TAB (tape automated bonding) tape carrier used for the mounting of a semiconductor element and an electrode thereof, an also to bonding tools used for the bonding methods.
FIG. 1 shows a conventional method of bonding an inner lead 13 of a tape carrier with a bonding pad 12 for a semiconductor element 15. In this method, a projection or bump 20 is formed on either the inner lead end 13 or the bonding pad 12, and the two members are bonded together via the bump 20. The bonding is effected using a bonding tool 21 as shown in FIG. 2A or 2B. The bonding tool 21 has a flat free end face, which has a size usually greater than the width of the inner lead 13 and almost equal to the size of the bonding pad 12. FIG. 2A shows a typical bonding tool 21a having a flat free end face, and FIG. 2B shows another typical bonding tool 21b having a flat free end face with a small raised cross portion.
In the prior art method of bonding the TAB inner lead, the bump is necessary for the following two major reasons. Firstly, the edges of the semiconductor element bonding pad 12 is covered by a passivation film 14, which is raised with respect to the bonding pad 12 surface, as shown in FIG. 3A. Therefore, it is very difficult to make the inner lead 13 directly contact with the bonding pad 12 when using the prior art bonding pad. Although the inner lead 13 is not held in contact with the passivation film 14 in its orthogonal direction (not shown), because the aperture or window of the passivation film 14 which is exposed through the electrode surface 12 is usually made greater than the width of the inner lead 13, the inner lead 13 is held in contact with the passivation film 14 in its longitudinal direction as shown in FIG. 3A. Therefore, it is very difficult to place the inner lead 13 in direct contact with the bonding pad surface 12 by using the prior art bonding tool 21, which has an end face with equal level in size to the bonding pad. For this reason, it is necessary to provide either individual bonding pads 12 or individual inner lead 13 with projections or bumps 20.
The second reason is that even if the contact of leads 13 with bonding pads surface 12 could be obtained as shown in FIG. 3B, the method of directly connecting the inner lead 13 to the bonding pad 12 is liable to cause cracking or damage in the pad structure 12 during bonding process. The inner lead 13 is usually comprised of a copper core surrounded by a gold or tin covering. Copper is hard and less capable of deformation compared to gold, which is a regular material of the bump. Therefore, when a bonding force is applied, it causes excessive stress concentration in the neighborhood of the contact area between the inner lead 13 edge and the bonding pad surface as shown in FIG. 3C. When ultrasonic vibration 18 is applied to a bonding tool 21 in this state, damage in the pad structure is liable to be produced in the neighborhood of the inner lead 13 edge where excessive stress concentration is produced.
Therefore, the bonding is normally effected via a bump 20 as shown in FIG. 1, which is soft like a gold bump to thereby avoid stress concentration, thus obtaining the bonding with sufficient strength without generation of any damage in the electrode or pad structure 12. This is so because the soft bump 20 is deformed before the applied pressure reaches a great value able to generate cracking in the pad structure 12.
The bumps may be formed on the bonding pads for a semiconductor element by various ways. For instance, a projection or bump is formed directly on the bonding pad by using electroplating technique, as disclosed in "ISHM'87 Proceedings", 1987, pp449-456 or "ISHM'88 Proceedings", 1988, pp117-124, or by a pedestal process using photoengraving techniques, in which the inner lead is half etched while masking a bump area, thus forming a raised portion or bump with a height of 30 to 40.mu., as disclosed in "1991 JAPAN IBMT SYMPOSIUM", WB2-02, 1991, pp81-84, or by a transfer bump process, in which a bump is formed on a glass substrate and then transferred onto a bonding pad, as disclosed in "IMC 1988 Proceedings", 1988, pp440-443.
In a further conventional process as shown in FIG. 4, an inner lead 13 is directly connected to a bonding pad for a semiconductor element 12 without formation of any bumps but by a thermosonic process like wire bonding process, as disclosed in U.S. Pat. No. 4,842,662. The process of directly bonding the inner lead to the bonding pad for the semiconductor element without formation of any bumps will hereinafter be referred to as direct bonding process. In the prior art direct bonding process disclosed in U.S. Pat. No. 4,842,662, the bonding is effected by using a bonding tool having a greater end face than the width of the inner lead and applying ultrasonic vibration thereto in the longitudinal direction. Further, the passivation film aperture over the bonding pad for the semiconductor element 12 is greater than the width of the inner lead 13.
As shown above, in the prior art method of bonding the TAB inner lead, it is normally necessary to form a bump on either a bonding pad for a semiconductor element or an inner lead. The bump formation usually requires a complicated electroplating method or like process as well as expensive equipment. Besides, it is difficult to attain 100 per cent bump formation yield rate. Further, in the bump formation process, damage to expensive semiconductor element or expensive TAB tape is liable. For the above reasons, the prior art TAB inner lead bonding process requiring the bump formation process leads to high assembly cost.
Furthermore, the other prior art of direct bonding process, using a bonding tool whose head is greater in end size than the width of the inner lead, is prone to cause any cracking in the pad structure, such as passivation cracking and substrate cracking under the bonding pad. This is thought to be attributable to the following cause. Referring to FIG. 3C, the P.sub.min value is defined as the minimum pressure value on the bond area, which can attain bonding an inner lead directly with a bonding pad, and the P.sub.max value is defined as the maximum pressure value on the bond area, which would not cause any cracking in the pad structure but could attain bondly an inner-lead directly with a bonding pad. In order to avoid crack or like damage to the pad structure 12, it is necessary to apply a bond force to the bonding tool 21 such that the effective pressure in the bond area 17 is lower than P.sub.max. The bonding is not effected in a region under a pressure lower than P.sub.min, because the inner lead 13 and the bonding pad 12 is in poor contact with each other. For obtaining the bonding while avoiding the generation of cracking in the pad strucutre, it is necessary that the actual pressure between the inner lead and the bonding pad is lower than P.sub.max and also that the region under pressure of P.sub.min is as small as possible. To this end, it is important that the pressure distribution over the bond area is as uniform as possible. In the prior art direct bonding process, however, stress concentration occurs in the neighborhood of the inner lead 13 edge and it leads to crack generation. To prevent the crack generation, it is necessary to reduce either the bond force or the ultrasonic energy applied to a bond tool. However, it has been difficult to obtain bonding with sufficient strength without causing any cracking in the pad structure.
Moreover, in the prior art direct bonding process, it is difficult to obtain good bonding properties, such as strong bond strength with all over the connections, in case of a semiconductor element having all the four sides thereof formed with bonding pads. The prior art bonding can be effected with a semiconductor element having only few opposed sides formed with bonding pads by applying ultrasonic vibration in the longitudinal direction thereof.